During the proposed three year period we intend to investigate the scope and limitations of mercury-mediated cyclopropane cleavages in organic synthesis. We will focus on the regio- and stereocontrol provided by neighboring group participation of approximate internal nucleophilic functionalities. The work proposed is not targeted towards any specific natural product. Part of our efforts will be aimed at completing studies already underway designed to elucidate the basic factors that affect the stereo-, regio-, and chemoselectivities inherent to lactonizations of cyclopropyl acid derivatives. Turning to alternative nucleophilic moieties, we will further look into the synthetic potential of these assisted cyclopropane cleavages. An unusual, potentially general means of achieving relative stereocontrol may provide added versatility. Many of the specific examples cited will illustrate potential utility in preparing conformaionally ill-defined, acyclic fragments characteristic of those found in the polyether and macrolide antibiotics.